FIG. 1 shows one example of a conventional current source according to the prior art. The collector of a first NPN transistor Q1 is coupled to a relatively positive voltage reference V.sub.CC via collector resistor R1. The emitter of transistor Q1 is coupled to ground through R2. Ground acts as a second voltage reference V.sub.EE. The base of transistor Q1 is connected to the base of a second NPN transistor Q2. The emitter of Q2 is coupled to V.sub.EE /ground through another emitter resister R3. An optional reference resistor R4 may be used to connect the bases of Q1 and Q2 to V.sub.EE /ground. Emitter following feedback transistor Q3 couples the collector of Q1 back to the base of Q1. The collector of Q3 is connected to V.sub.CC. The output current is provided at the collector of Q2.
If the assumption is made that transistors Q1 and Q2 have equal V.sub.BE 's, and the further simplifying assumption is made that the beta of all of the transistors are equal to infinity, then the currents I.sub.REF and I.sub.OUT, flowing respectively through transistors Q1 and Q2, are determined as follows: ##EQU1##
As can be seen from these relationships, the output current is directly dependent on V.sub.BE, since the reference current I.sub.REF is dependent on V.sub.BE and the output current I.sub.OUT is proportional to the reference current I.sub.REF. It would be very desirable to eliminate this dependence without adding any complex or expensive circuitry.
U.S. Pat. No. 4,714,872 to Traa for a "Voltage Reference for Transistor Constant Current Source", hereby incorporated by reference, discloses a voltage reference circuit for a constant current source bipolar transistor. One of the two voltages produced by this circuit varies with the negative temperature coefficient, i.e., V.sub.BE, of a bipolar transistor. The other voltage produced by this circuit has a constant magnitude that is independent of supply variations. The reference voltages produced by this circuit thus allow a constant current source to operate independently of both temperature variations and supply variations. However, this circuit is quite complex and therefore relatively expensive, thus limiting its usefulness for some applications.
U.S. Pat. No. 4,792,748 to Thomas et al. for a "Two-Terminal Temperature-Compensated Current Source Circuit", hereby incorporated by reference, discloses a reference current source in which plus and minus temperature correction currents are summed to produce a final reference current whose temperature dependence can be selected to be zero or a range of plus or minus values, depending on the selection of two resistor values. U.S. Pat. No. 4,460,865 to Bynum et al. for "Variable Temperature Coefficient Level Shifting Circuit and Method", hereby incorporated by reference, discloses a similar approach.